Method for extruding a polymeric material and extrusion head therefor

ABSTRACT

An apparatus for extruding a polymeric material, having an extrusion head which includes a male die, a female die coaxially arranged with respect to the male die, a conveying channel, and at least one portion of which is defined between the male die and the female die. The apparatus further includes a device for adjusting a cross-sectional area of the at least one portion of the conveying channel by reciprocally displacing the female die with respect to the male die in response to an extrusion speed variation of the polymeric material.

The present invention relates to a method for extruding a polymericmaterial.

In particular, the present invention relates to a method for extrudingan elastomeric material to be used in tyre manufacturing processes.

The invention further relates to an extrusion head for extruding apolymeric material and to an extrusion apparatus which comprises saidextrusion head.

A preferred field of application of the present invention is a processfor manufacturing a tyre wherein elastomeric sheets or strip-likeelements, possibly reinforced with metallic or synthetic thread-likeelements, are used for producing tyre constitutive elements such as, forexample, a carcass ply, a belt layer, a sidewall, a bead core.

A tyre generally comprises: a carcass structure comprising at least onecarcass ply, the ends of which are folded back or secured to two annularreinforcing elements, i.e. the so-called “bead cores”; a tread band; abelt structure placed between the carcass structure and the tread band;and a pair of sidewalls applied to said carcass structure in axiallyopposite positions.

The tyre portion which comprises the bead core is known as “bead” andperforms the function of fixing the tyre on a respective mounting rim.

Generally, in a position radially external to said bead core, the beadcomprises a rubber strip, conventionally called “bead filling” or “beadapex”, which has a substantially triangular cross-section and extendsradially outwardly from the respective bead core.

In conventional manufacturing processes, the tyre constitutive elementsare made by using semi-finished products, i.e. continuous sheets ofelastomeric material—possibly in combination with reinforcing elementssuch as steel or textile cords—that are prepared separately and in largequantities previously to the tyre assembling operations.

According to said conventional processes, for each tyre constitutiveelement, the manufacturing process comprises the steps of winding apredetermined elastomeric sheet onto a building drum, cutting (or insome cases pre-cutting) said sheet into a length approximately equal tothe circumference of the drum, and joining the circumferentiallyopposite ends of said sheet length directly on the building drum.

In more recent times particular attention has been given to productionmethods that eliminate or at least remarkably reduce the preliminaryproduction of said semi-finished products. For example, European patentN. 928,680 in the name of the same Applicant discloses the manufacturingof a green tyre by consecutively producing and assembling together on atoroidal support the tyre constitutive elements. In detail, the tyre ismanufactured by axially overlapping and/or radially superimposing turns(coils) of a strip-like element on the toroidal support, said strip-likeelement being a strip of an elastomeric material only, or a strip ofelastomeric material embedding reinforcing elements thereinto, typicallytextile or metal cords, or a rubberized metal wire or cord.

According to said further process, the toroidal support is moved,preferably by a robotized system, between a plurality of work stationsin each of which, through automated sequences, a particular buildingstep of the tyre is carried out.

The manufacturing process further comprises the successive step ofmoulding the green tyre, so as to confer to the latter a desired treadpattern, and the step of curing the green tyre, so as to confer to thelatter a desired geometrical conformation which is obtained by curingthe elastomeric material forming the tyre.

The moulding and curing steps of the green tyre are carried out byintroducing the green tyre into a moulding cavity defined within avulcanization mould, whose shape matches the shape of the outer surfaceof the tyre to be obtained, and by introducing a fluid under pressureinto a diffusion interspace (or diffusion gap) provided between theinner circumferential surface of the green tyre and the toroidalsupport. Such a tyre manufacturing process is described, for instance,in European Patent EP-976,533 in the name of the same Applicant.

In the following description, the term “extruded element” is used toindicate either a strip-like element or a sheet. In particular, the term“extruded element” is used to indicate a strip-like element or a sheetwhich is made of a polymeric material or which is reinforced with atleast one thread-like element. The term “thread-like element” is used toindicate an element whose longitudinal dimension is substantiallygreater than its transversal dimension, the thread-like elementcomprising one or more reinforcing elements, each reinforcing elementconsisting of a textile or metallic filament or cord. In case thestrip-like element or the sheet is made of a polymeric material, thestrip-like element or the sheet is preferably obtained by extrusion. Incase the strip-like element or the sheet is reinforced with at least onethread-like element, the strip-like element or the sheet is preferablyobtained by extruding a polymeric material onto at least one thread-likeelement advancing through an extrusion apparatus.

Generally, an extrusion apparatus comprises an extrusion head whichincludes: a male die; a female die, coaxially arranged with respect tothe male die; and a distributor element for uniformly distributing theextruded material into a conveying channel which is provided between themale die and the female die.

In case the extruded element is reinforced with at least one thread-likeelement, the male die is usually provided with an inner cavity coaxiallyextending with respect to a longitudinal axis of the extrusion head,said cavity being suitable for receiving said at least one thread-likeelement advancing along a direction substantially parallel to saidlongitudinal axis. The polymeric material flowing into the conveyingchannel is thus deposited onto said at least one thread-like elementadvancing through the extrusion apparatus.

Document U.S. Pat. No. 3,752,614 discloses an extrusion head for forminginsulated wire which includes a fixed threaded hollow mandrel and athreaded hollow pin disposed internally of, and in mating engagementwith, the mandrel for supporting a male die member in axial alignmentwith a female die member mounted within the head. The threaded portionsof the mandrel and the pin are so engaged that rotation of the pinwithin the mandrel advances or retracts the male die member with respectto the female die member while maintaining the alignment therebetween.This device allows to compensate for changes in the extruded plasticmaterial, insulation thickness, or in the pressure or temperature of thesystem while the extrusion head is in operation. The relative movementbetween the male die member and the female die member is effected by anoperator by manual control.

Document U.S. Pat. No. 3,583,033 discloses a die for in-line extrusionof viscoelastic and viscous thermoplastic materials, comprising aconical male valve member which is advanced or retracted with respect toa conical seat to vary the degree of shear and back pressure to whichthe material is exposed in passing through the annular conicalpassegeway. The movement of the conical male valve member is achieved byrotating a ring nut and is manually effected and controlled by anoperator.

Document GB-2,060,473 discloses a head for extruding tubes for blowmoulding, including a mandrel supported by one part, the other partcomprising at least one conical wall portion which, with a correspondingmandrel wall portion, forms a conical flow space section whosethroughflow cross-section can be varied by the relative displacement ofthe two telescopically engaging parts. The relative displacement ismanually effected by the operator by means of an adjusting screwassociated with one part of the mandrel and engaged with a suitablescrewthread formed on the other part of the mandrel. It is describedthat various remote controlled servo devices could conceivably be usedin place of the adjusting screw.

Document U.S. Pat. No. 3,402,427 discloses a crosshead die bodyapparatus including a shaping die for extruding and shapingthermoplastic material comprising polyvinylidene fluoride resin, whereinthe crosshead die body has at least two externally adjustable internallyaxially positioned frusto-conically shaped valving means and at leastone annular orifice portion of fixed uniform annular width and ofsubstantially fixed but adjustably variable length located axiallybetween said valving means, whereby the pressure drop and shearingstress between the extruder outlet and the shaping die may beprogressively and precisely controlled. During start-up of the coatingprocess, the surface and body characteristics of the extrudate areobserved by the operator of the machine and are modified by manipulationof the valving means until the optimum extrudate characteristic areobtained. Then, by continued observation and manipulation of the valvingmeans, either by manual or automatic control, the optimumcharacteristics can be maintained by the operator throughout theextruding and shaping operation without reaching or exceeding the yieldpoint of the resin.

The Applicant has noted that in case the step of extruding a polymericmaterial to obtain an extruded element is part of a complex process,such as a tyre manufacturing process, generally the extrusion speed ofsaid material needs to be regulated on the basis of the process stepsubsequent to the extrusion step, while the extrusion apparatus iscontinuously fed with the polymeric material to be extruded.

In particular, in the more recent tyre manufacturingprocesses—disclosed, for instance, in European patent N. 928,680mentioned above—wherein the tyre constitutive elements are produced andconsecutively assembled on a toroidal support while the tyre is beingmanufactured so that the storage of semi-finished products issubstantially avoided, the Applicant has noted that the extrusion speedneeds to be increased when the extruded elements—which are used forforming a specific tyre component—have to be obtained and applied ontothe tyre being manufactured, and, on the contrary, the extrusion speedneeds to be decreased when the extruded elements do not have to bedeposited so that waste of raw materials is avoided or at leastremarkably reduced.

The Applicant has noted that the regulation of the extrusion speed hasto be carried out while ensuring that the extruded elements maintain thedesired quality.

In fact, the Applicant has noted that the quality of the extrudedelements remarkably influences the quality of the tyre reinforcingstructures including said extruded elements. Therefore, great attentionand care have to be paid to the production of the extruded elements incase the latter are reinforced with thread-like elements.

In particular, uniformity and homogeneity of the extruded material inthe production of the extruded elements have to be carefully controlledand a substantially constant thickness thereof has to be ensured so thatuniformity of the tyre is guaranteed. Moreover, in case the extrudedelements are reinforced with thread-like elements, great attention andcare have to be paid in order to obtain extruded elements wherein thepolymeric coating layer uniformly adheres to the thread-like elements sothat the thickness of the coating layer is substantially constant alongthe longitudinal extension of the extruded elements.

The Applicant has noted that the quality of the extruded elementsdepends on the geometry of the extrusion head and, at a predeterminedgeometry thereof, on the physico-chemical characteristics of theextruded material as well as on the process parameters of the extrusionprocess.

For example, particular care is required when polymeric materials thatare sensitive to temperature are used. In fact, scorching at hightemperatures and/or clots at low temperatures can arise, principally iflong periods of permanency thereof are caused to occur in the extrusionhead. Scorching and clots need to be avoided since they negativelyinfluence uniformity and homogeneity of the extruded elements, and thusof the tyre reinforcing structures including said extruded elements.Moreover, said defects can cause the extrusion process to be stopped inorder to allow the extrusion head to be cleaned from the clots and/orthe scorched material.

Furthermore, the Applicant has noted that stagnation zones of thepolymeric material can originate in the extrusion head, particularlywhen small flow rate values of the polymeric material occur, forinstance when the output of the extrusion process is decreased. As aconsequence, the period of permanency of the polymeric material in theextrusion head increases and, as mentioned above, scorching oroverheatings of said material can occur.

Moreover, the Applicant has noted that, at a predetermined and constantextrusion speed of the extruded elements, a remarkable decrease of theflow rate of the polymeric material flowing through the extrusion headcan also cause the formation of very thin extruded elements and,sometimes, can even cause the breakage thereof. In case the extrudedelements are reinforced with thread-like elements, areas of the outersurface thereof can be even devoid of the extruded material. On thecontrary, at a predetermined and constant extrusion speed of theextruded elements, a remarkable increase of the flow rate of thepolymeric material flowing through the extrusion head can cause theformation of too thick extruded elements.

The Applicant has noted that a variation of the extrusion speed, whichis chosen in response to the process step subsequent to the extrusionstep, causes a variation of the flow rate of the extruded polymericmaterial and, consequently, a variation of the pressure inside theextrusion head.

In the present description and in the following claims the term“extrusion speed” is used to indicate the linear velocity of thepolymeric extruded element exiting from the extrusion head. In case theextruded element consists only of the polymeric material (i.e. theextruded element does not embed any reinforcing element), the extrusionspeed can be varied by modifying the rotational speed of the extruderscrew. In case the extruded element comprises at least one thread-likeelement embedded in the polymeric material, the extrusion speed can begenerally varied by modifying the linear velocity of the at least onethread-like element.

In particular, the Applicant has noted that an increase of the extrusionspeed requires an increase of the flow rate of the extruded polymericmaterial and thus causes an increase of the pressure inside theextrusion head. Therefore, in order not to mechanically stress theextrusion head and not to scorch the polymeric material, the flow rateof the polymeric material needs to be kept below a predetermined maximumvalue, fact which inevitably limits the maximum extrusion speed value.On the other side, a decrease of the extrusion speed requires a decreaseof the flow rate of the extruded polymeric material and thus causes adecrease of the pressure inside the extrusion head. As a consequence,the period of permanency of the polymeric material in the extrusion headincreases and, as mentioned above, scorching of the polymeric materialas well as formation of stagnation zones can occur. Therefore, the flowrate of the polymeric material needs to be kept over a predeterminedminimum value, fact which inevitably limits the minimum extrusion speedvalue.

The Applicant has perceived the need of increasing the operative rangeof the flow rate of the polymeric material to be extruded so that therange of variation of the extrusion speed can be remarkably increasedand can be suitably fitted to the working conditions of the process stepsubsequent to the extrusion step.

In particular, the Applicant has perceived that the above goal can beachieved by modifying the geometry of the extrusion head during theextrusion process, the variation of geometry of the extrusion head beingcarried out in response to the working conditions of the process stepsubsequent to the extrusion step.

In detail, the Applicant has found that, once the desired range ofvariation of the extrusion speed has been chosen on the basis of theworking conditions of the process step subsequent to the extrusion step,the extrusion head can operate at the corresponding operative range ofthe flow rate of the polymeric material to be extruded by adjusting thecross-sectional area of the conveying channel in response to the actualflow rate flowing through the extrusion head, and thus in response tothe actual extrusion speed which is required in the specific phase ofthe extrusion process.

In a first aspect the present invention relates to a method forextruding a polymeric material, said method comprising the steps of:

-   -   feeding said polymeric material to an extrusion apparatus        including an extrusion head, said extrusion head comprising:        -   a male die;        -   a female die, coaxially arranged with respect to said male            die, and        -   a conveying channel, at least one portion of which being            defined between said male die and said female die;    -   adjusting a cross-sectional area of said at least one portion of        the conveying channel by reciprocally displacing said female die        with respect to said male die in response to an extrusion speed        variation of said polymeric material.

In accordance with the present invention, since the cross-sectional areaof the conveying channel is adjusted in response to the variation of theextrusion speed, critical flow conditions can be avoided and thusscorching or overheating of the polymeric material as well as theformation of stagnation zones in the extrusion head or mechanicaldamages thereof can be avoided or at least substantially reduced.

In other words, according to the present invention the geometry (i.e.the cross-sectional area) of the conveying channel can be automaticallyadapted to the different flow conditions of the polymeric material bymeans of a reciprocal displacement of the female die with respect to themale die.

In fact, according to the present invention, an increase of theextrusion speed requires a corresponding increase of the flow rate ofthe polymeric material to be extruded and thus causes a correspondingincrease of the pressure in the extrusion head. The resulting pressureincrease has the effect of axially displacing the female die from themale die so that the cross-sectional area of the conveying channelincreases and the pressure losses in the conveying channel decrease. Asa result of the conveying channel cross-section variation, the pressureincrease in the extrusion head can be limited and the desired flow ratevalue of the polymeric material can be guaranteed. On the contrary, adecrease of the extrusion speed requires a corresponding decrease of theflow rate of the polymeric material to be extruded and thus causes acorresponding pressure decrease and an increase of the period ofpermanency of the polymeric material in the extrusion head. Theresulting pressure decrease has the effect of axially moving the femaledie towards the male die so that the cross-sectional area of theconveying channel decreases and the pressure losses in the conveyingchannel increase. As a result of the conveying channel cross-sectionvariation, the period of permanency of the polymeric material in theextrusion head can be suitably controlled and the desired flow ratevalue of the polymeric material can be guaranteed.

According to the method of the present invention, the step of adjustingthe cross-sectional area of at least one portion of the conveyingchannel by reciprocally regulating the position of the female die withrespect to the male die comprises the step of partially counteractingthe force exerted on at least one die by the polymeric material flowingin the conveying channel.

Preferably, the step of adjusting comprises the step of partiallycounteracting the force exerted on the female die by the polymericmaterial flowing in the conveying channel.

Preferably, the counteracting force is substantially parallel to theextrusion head longitudinal axis.

According to an embodiment of the present invention, the step ofpartially counteracting the force exerted on at least one die is carriedout by means of a resilient element acting on said at least one diealong said longitudinal axis.

Preferably, said resilient element is associated with the at least onedie which is allowed to be axially displaced.

Preferably, said resilient element is a spring whose elastic constant Kis selected in response to a predetermined range of variation of theextrusion speed.

The method of the present invention further comprises the step ofextruding the polymeric material.

The method of the present invention further comprises the step ofchoosing the extrusion speed variation of the polymeric material inresponse to working conditions of a process step subsequent to theextrusion step.

According to a further embodiment, the method of the present inventionfurther comprises the steps of:

-   -   detecting a variation of at least one parameter indicative of        the polymeric material flow conditions, the variation of said at        least one parameter being associated to the extrusion speed        variation of said polymeric material, and    -   adjusting the cross-sectional area of at least one portion of        the conveying channel in response to the detected variation of        said at least one parameter.

Preferably, said at least one parameter indicative of the polymericmaterial flow conditions is the pressure. Preferably, the pressure isdetected in the extrusion head.

Preferably, said at least one parameter indicative of the polymericmaterial flow conditions is detected with a predetermined frequencyvalue. Alternatively, said at least one parameter is continuouslydetected.

Preferably, the step of detecting the variation of at least oneparameter indicative of the polymeric material flow conditions comprisesthe step of generating a signal representative of said variation bymeans of a sensor acting on said polymeric material flowing through theextrusion head.

Preferably, the step of adjusting the cross-sectional area of theconveying channel comprises the steps of:

-   -   calculating a second position of at least one die in response to        a variation of said extrusion speed occurring at a first        position, and    -   moving said at least one die to said second position.

Preferably, the step of calculating comprises the step of calculatingthe second position of the female die and the step of moving comprisesthe step of moving the female die to said second position.

Preferably, the step of moving said at least one die to said newposition is carried out by means of an actuator device. Preferably, theactuator device is associated with said at least one die and saidsensor.

Preferably, the method of the present invention is suitable forextruding a cross-linkable material, the latter being particularlysensitive to temperature variations.

In a second aspect thereof, the present invention relates to anapparatus for extruding a polymeric material, said apparatus includingan extrusion head which comprises:

-   -   a male die;    -   a female die, coaxially arranged with respect to said male die;    -   a conveying channel, at least one portion of which being defined        between said male die and said female die, and    -   a device for adjusting a cross-sectional area of said at least        one portion of said conveying channel by reciprocally displacing        said female die with respect to said male die in response to an        extrusion speed variation of said polymeric material.

Preferably, the male die is provided with an inner cavity coaxiallyextending with respect to a longitudinal axis of the extrusion head,said cavity being suitable for receiving at least one thread-likeelement which is used for reinforcing an extruded element.

In a first embodiment of the extrusion apparatus of the presentinvention, the device for adjusting the cross-sectional area of saidconveying channel comprises a resilient element which acts on at leastone die and partially counteracts the force exerted on said at least onedie by the polymeric material flowing in the conveying channel.

Preferably, said resilient element is associated with the female die.

Preferably, said resilient element is a spring whose elastic constant Kis selected in response to the desired range of variation of theextrusion speed.

According to a further embodiment of the extrusion apparatus of thepresent invention, the device for adjusting the cross-sectional area ofsaid conveying channel is a servo-device for detecting the variation ofat least one parameter indicative of the polymeric material flowconditions (the variation of said at least one parameter beingassociated to the extrusion speed variation of said polymeric material)and for adjusting said cross-sectional area on the basis of the detectedvariation of said at least one parameter.

Preferably, said servo-device comprises a sensor acting on saidpolymeric material and generating a signal representative of thevariation of said at least one parameter.

Preferably, said sensor detects a pressure variation of the polymericmaterial flowing through the extrusion head.

Preferably, said servo-device further comprises a device for calculatinga new position of at least one die and a device for moving said at leastone die to said new position.

Preferably, the device for moving said at least one die to said newposition is an actuator device. Preferably, said actuator device isassociated with said at least one die and a position sensor. Theposition sensor has the function of detecting the position of said atleast one die.

According to a preferred embodiment, said actuator device comprises ahydraulic device.

According to a further embodiment, said actuator device comprises apneumatic device.

According to a further embodiment, said actuator device comprises a gearelectro-mechanical device.

According to a further embodiment, said actuator device comprises alinear actuator.

In a third aspect thereof, the present invention relates to a processfor manufacturing a tyre, said process comprising the steps of:

-   -   forming a crude tyre on a supporting device;    -   moulding said crude tyre, and    -   curing said crude tyre,        wherein the step of forming the crude tyre comprises the step of        extruding at least one elastomeric material, the step of        extruding comprising the steps of:    -   feeding said elastomeric material to an extrusion apparatus        including an extrusion head, said extrusion head comprising:        -   a male die;        -   a female die, coaxially arranged with respect to said male            die, and        -   a conveying channel, at least one portion of which being            defined between said male die and said female die;    -   adjusting a cross-sectional area of said at least one portion of        the conveying channel by reciprocally displacing said female die        with respect to said male die in response to an extrusion speed        variation of said elastomeric material.

Preferably, the step of forming the crude tyre comprises the step ofextruding at least one elastomeric material in the form of a strip-likeelement to be deposited onto the crude tyre being manufactured. In thiscase, preferably the supporting device is a toroidal support.

Alternatively, the step of forming the crude tyre comprises the step ofextruding at least one elastomeric material in the form of a sheet to bedeposited onto the crude tyre being manufactured. In this case,preferably the supporting device is a building drum.

Throughout the present description and the following claims, the term“elastomeric material” is used to indicate a composition comprising atleast one elastomeric polymer and at least one reinforcing filler.Preferably, the composition further comprises additives, such as across-linking agent and/or a plasticizer.

The method of the present invention can be advantageously used eitherfor extruding a semi-finished product (in the form of a sheet) to beused in a conventional tyre manufacturing processes, or for extrudingstrip-like elements which are employed in more recent tyre manufacturingprocesses as described above.

However, the present invention can be also applied to technical fieldsdifferent from a tyre manufacturing process. In particular, the presentinvention can be applied to any technical field wherein the extrusion ofa polymeric material—to obtain an extruded element—is required.

Further characteristics and advantages of the present invention willbecome clearer from the description made hereafter with reference to theattached drawings in which, for illustrative and non limiting purposes,four embodiments of an extrusion head for carrying out the method of thepresent invention are shown. In the drawings:

FIG. 1 is a schematic cross-sectional view of a first embodiment of anextrusion head in accordance with the present invention;

FIG. 2 is a schematic cross-sectional view of a second embodiment of anextrusion head in accordance with the present invention;

FIG. 3 is a schematic cross-sectional view of a third embodiment of anextrusion head in accordance with the present invention;

FIG. 4 is a schematic cross-sectional view of a fourth embodiment of anextrusion head in accordance with the present invention;

FIG. 5 is a graph showing the pressure variation of the polymericmaterial as a function of the flow rate thereof through an extrusionhead in accordance with the present invention in comparison with aconventional extrusion head provided with a fixed geometry of theconveying channel;

FIG. 6 is a graph showing the temperature variation of the polymericmaterial as a function of the flow rate thereof through an extrusionhead in accordance with the present invention in comparison with aconventional extrusion head provided with a fixed geometry of theconveying channel;

FIG. 7 is a graph showing the period of permanency of the polymericmaterial as a function of the flow rate thereof through an extrusionhead in accordance with the present invention in comparison with aconventional extrusion head provided with a fixed geometry of theconveying channel.

FIG. 1 schematically shows an extrusion head, indicated with referencenumber 1, for extruding a coating layer 100—made of a polymeric material2—at a radially outer surface of an elongated element 3 advancingthrough the extrusion head 1 along a direction indicated by arrow A.

The extrusion head 1 has a longitudinal axis X-X and is part of anextrusion apparatus which is not illustrated in detail as beingconventional per se.

As mentioned above, the extrusion head 1 of the present invention can beused in a process for manufacturing a tyre. In such a case, theelongated element 3 can be a metallic or synthetic thread-like elementwhich is covered by extrusion with an elastomeric material to form anextruded element to be used in the manufacturing of tyre reinforcingstructures, such as, for example, the carcass structure, the beltstructure, the “bead cores”.

According to the embodiment shown in FIG. 1, the extrusion head 1comprises: a distributor element 11, a male die 12, a female die 13 andan annular body 10. The annular body 10 is coaxially arranged withrespect to the distributor element 11, the male die 12 and the femaledie 13 and positioned radially external thereto.

In particular, the annular body 10 is provided with an inner cavity 14coaxially extending with respect to the longitudinal axis X-X andsuitable for housing the distributor element 11, the male die 12 and thefemale die 13.

The extrusion head 1 is further provided with an inlet duct 15 forfeeding the polymeric material 2. The inlet duct 15 is associated to theannular body 10 at a feeding duct 16 which is formed in the annular body10 and which extends, in the illustrated embodiment, in a directionsubstantially perpendicular to the axis X-X.

In a way known per se, for example through pipes not illustrated, theinlet duct 15 and the feeding duct 16 are in fluid communication with anextruder barrel provided with at least one extruder screw (notillustrated since conventional per se).

The distributor element 11 comprises a tubular body 17 on the outersurface of which is provided at least one pair of distribution channels18, only one of which being shown by a dashed line in FIG. 1.

The tubular body 17 of the distributor element 11 is provided, similarlyto the annular body 10, with an inner cavity 19 coaxially extending withthe longitudinal axis X-X and intended for receiving the elongatedelement 3 advancing along the direction A.

In operation, preferably the advancing direction A of the elongatedelement 3 is substantially parallel to the longitudinal axis X-X of theextrusion head 1.

The extrusion head 1 further comprises an annular conveying channel 20.A first portion 20′ of said conveying channel (i.e. the conveyingchannel portion positioned in proximity of the feeding duct 16) iscoaxially defined between a radially inner surface of the annular body10 and a radially outer surface of the tubular body 17 of thedistributor element 11. A second portion 20″ of said conveying channel(i.e. the conveying channel portion positioned in proximity of the exitof the extrusion head) is defined between the male die 11 and the femaledie 12.

The conveying channel 20 is used for conveying the polymeric material 2to be deposited onto the outer surface of the elongated element 3. Tothis purpose, the conveying channel 20 defines a substantially annularand continuous passageway which is coaxial with the longitudinal axisX-X.

The distribution channels 18 are formed on the outer surface of thetubular body 17 and each channel is in fluid communication with thefeeding duct 16.

In the embodiment illustrated in FIG. 1, the distribution channels 18have a development of a curvilinear type, preferably of helical type,and extend on radially opposite sides with respect to the longitudinalaxis X-X. Each distribution channel 18 carries out the function ofdistributing the polymeric material entering the inlet duct 15 as muchhomogeneously as possible in the conveying channel 20 so as to allow auniform production of the desired coating layer 100.

Similarly to the annular body 10 and the tubular body 17 of thedistributor element 11, the male die 12 and the female die 13 areprovided with an inner cavity 21 for allowing the elongated element 3 topass through, while the polymeric material 2—which flows in theconveying channel 20—is deposited on the outer surface of the elongatedelement 3.

In accordance with the present invention, the female die 13 is slidablyassociated with the annular body 10 so as to be axially movable withrespect to the male die 12.

According to the present invention, the provision of a female die 13which can be displaced with respect to the male die 12 allows thecross-sectional area of the second portion 20″ of the conveying channel20 to be modified during operation on the basis of the extrusion speedvariation of the polymeric material 2.

To this purpose, the extrusion head 1 comprises a device for adjustingthe cross-sectional area of the second portion 20″ of the conveyingchannel 20 on the basis of said extrusion speed variation. Inparticular, said device acts on the female die 13 to adjust the positionthereof with respect to the male die 12 along the longitudinal axis X-Xin response to the extrusion speed variation.

According to an alternative embodiment (not shown), a similar result canbe achieved by providing a device for adjusting the cross-sectional areaof the second portion 20″ of the conveying channel 20 which acts on amale die which is movable along the longitudinal axis X-X with respectto a stationary female die, i.e. to a female die that is in a fixedposition.

In the embodiment illustrated in FIG. 1, the device for adjusting theposition of the female die 13 with respect to the male die 12 along thelongitudinal axis X-X comprises a resilient element 22 which isinterposed between the female die 13 and a supporting element 23 atleast partially fitted onto the annular body 10 by any conventionalfastening means (not shown). The resilient element 22 illustrated inFIG. 1 is a spring. The supporting element 23 houses the spring 22 andis provided, at a free end thereof, with a passage 24 for allowing theelongated element 3, coated with the polymeric material 2, to advancealong the direction A and to come out from the extrusion head 1.

The value of the elastic constant K of the resilient element 22 iscalculated in such a way that the rigidity thereof can at leastpartially counteract the force exerted on the female die 13 by thepolymeric material 2 flowing through the conveying channel 20 whateveris the extrusion speed (and thus the flow rate) of said material, saidextrusion speed being comprised in the desired extrusion speedvariation.

With reference to the embodiment of the extrusion head described aboveand illustrated in FIG. 1, the method according to the present inventionfor depositing by extrusion a polymeric material 2 on an elongatedelement 3 advancing in the extrusion head 1 along a direction A toobtain the coating layer 100 comprises the following steps.

In a first step, after having conveyed the elongated element 3 withinthe longitudinal cavity 19 of the extrusion head 1, the polymericmaterial 2 is fed into the feeding duct 16 of the extrusion head throughthe inlet duct 15. The polymeric material 2 is caused to flow into theconveying channel 20 through the distribution channels 18.

In a second step, the force exerted by the polymeric material 2 on thefemale die 13 is at least partially counteracted by the elastic forceexerted by the spring 22 which allows an adjustment of thecross-sectional area of the second portion 20″ of the conveying channel20. Therefore, according to the present invention, the cross-sectionalarea of the second portion 20″ of the conveying channel 20 isautomatically regulated by adjusting the position of the female die 13with respect to the male die 12 on the basis of the actual value of thedesired extrusion speed (and thus of the resulting flow rate of thepolymeric material 2 flowing through the conveying channel 20).

Therefore, in operation, it is advantageously possible to increase theflow rate of the polymeric material 2 flowing in the extrusion head 1,for example in order to increase the extruded element production yield,while ensuring at the same time that the values of pressure, temperatureand period of permanency do not cause mechanical damages to theextrusion head 1 as well as scorching or overheating of the polymericmaterial.

This aspect is shown in detail in the graphs reported in FIGS. 5 and 6,wherein the variation of pressure and temperature, respectively, of thepolymeric material as a function of the flow rate thereof through anextrusion head in accordance with the present invention in comparisonwith a conventional extrusion head provided with a fixed geometry of theconveying channel are shown.

With reference to the graph shown in FIG. 5, the variation of pressureas a function of the flow rate at three different positions (X₁, X₂, X₃)of the female die 13 along the longitudinal axis X-X is shown. Indetail, the value of position X₁ is smaller than the value of positionX₂ and the value of position X₂ is smaller than the value of position X₃(i.e. X₁<X₂<X₃) while considering the female die axially movingaccording to the direction A, i.e. axially departing from the male die,so that the cross-sectional area of the second portion 20″ of theconveying channel is caused to increase.

For each position of the female die, the variation of pressure as afunction of flow rate (i.e. the pressure/flow rate curves indicated withreferences a, b, c respectively) is obtained by varying the flow ratevalue of the polymeric material and measuring the corresponding pressurevalue at the inlet duct of the extrusion head by means of a pressuresensor. In the case a conventional extrusion head (which is providedwith a fixed geometry of the conveying channel, i.e. the female die andthe male die are not reciprocally displaceable) is considered, thestationary female die of which being located at the position X₁, anincrease of the flow rate of the polymeric material from Q₁ to Q₂ causesa corresponding increase of the pressure from P₁ to P₂. In fact, sincethe female die is stationary at the position X₁, the only possible pathfor passing from Q₁ to Q₂ is along curve a.

On the contrary, in the extrusion head of the present invention, anincrease of the flow rate from Q₁ to Q₂ causes a displacement of thefemale die position from X₁ to X₃ so that it is possible to obtain theflow rate Q₂ at a pressure value P′₂ which is smaller than P₂ (thepressure value P₂ corresponds to the flow rate value Q₂ while movingalong curve a, i.e. in the presence of a conventional extrusion head)since, according to the present invention, the path for passing from Q₁to Q₂ is along curve S. The extremes of curve S are used to calculatethe elastic constant K of the resilient element 22.

Similarly, with reference to the graph shown in FIG. 6, the variation ofthe temperature as a function of the flow rate at three differentpositions (X₁, X₂, X₃) of the female die 13 along the longitudinal axisX-X is shown. The female die axially moves according to the direction A,i.e. axially departs from the male die, so that the cross-sectional areaof the second portion 20″ of the conveying channel is caused toincrease.

For each position of the female die, the variation of temperature as afunction of flow rate (i.e. the temperature/flow rate curves indicatedwith references d, e, f respectively) is obtained by varying the flowrate value of the polymeric material and measuring the correspondingtemperature value at the inlet duct of the extrusion head by means of atemperature sensor. In the case a conventional extrusion head (which isprovided with a fixed geometry of the conveying channel, i.e. the femaledie and the male die are not reciprocally displaceable) is considered,the stationary female die of which being located at the position X₁, anincrease of the flow rate of the polymeric material from Q₁ to Q₂ causesa corresponding increase of the temperature from T₁ to T₂. In fact,since the female die is stationary at the position X₁, the only possiblepath for passing from Q₁ to Q₂ is along curve d.

On the contrary, in the extrusion head of the present invention, anincrease of the flow rate from Q₁ to Q₂ causes a displacement of thefemale die position from X₁ to X₃ so that it is possible to obtain theflow rate Q₂ at a temperature value T′₂ which is smaller than T₂ (thetemperature value T₂ corresponds to the flow rate value Q₂ while movingalong the curve d, i.e. in the presence of a conventional extrusionhead) since, according to the present invention, the path for passingfrom Q₁ to Q₂ is along curve S′. The extremes of curve S′ are used tocalculate the elastic constant K of the resilient element 22.

Thus, with reference to the embodiments of the extrusion head 1described above, the present invention allows to increase the flow rateof the polymeric material 2 flowing through the extrusion head 1 whileensuring at the same time that the values of pressure and temperatureremain within an acceptable range of values, so as to avoid thatcritical flow conditions in the extrusion head can occur.

On the other hand, the present invention makes also advantageouslypossible to decrease the flow rate of the polymeric material in theextrusion head while ensuring, at the same time, that the values ofpressure and period of permanency of the polymeric material remainwithin respective acceptable range of value, so as to avoid theformation of stagnation zones as well as scorching or overheating of thematerial being extruded.

This aspect is shown in the graph illustrated in FIG. 7, wherein theperiod of permanency of the polymeric material as a function of the flowrate at three different positions (−X₁, −X₂, −X₃) of the female die 13along the longitudinal axis X-X is shown. The three different positionsare indicated with negative values since in this case the movement ofthe female die with respect to the male die occurs in a directionopposite to that of arrow A; in fact, the cross-sectional area of thesecond portion 20″ of the conveying channel is caused to decrease.

For each position of the female die, the variation of the period ofpermanency as a function of the flow rate (i.e. the period ofpermanency/flow rate curves indicated with references g, h, irespectively) is obtained by varying the flow rate value of thepolymeric material and calculating the corresponding period ofpermanency in the extrusion head. In the case a conventional extrusionhead (which is provided with a fixed geometry of the conveying channel,i.e. the female die and the male die are not reciprocally displaceable)is considered, the stationary female die of which being located at theposition −X₁, a reduction of the flow rate of the polymeric materialfrom Q₁ to Q₂ causes a corresponding increase of the period ofpermanency from t₁ to t₂. However, the value t₂ is quite close to thecritical value t_(scorch,2) which represents the period of permanency atwhich scorching of the polymeric material occurs (indicated by the curvet_(scorch)). In fact, since the female die is stationary at the position−X₁, the only possible path for passing from Q₁ to Q₂ is along curve g.

On the contrary, in the extrusion head of the present invention, areduction of the flow rate from Q₁ to Q₂ causes a displacement of thefemale die position from −X₁ to −X₃ so that it is possible to obtain theflow rate Q₂ at a period of permanency t′₂ which is smaller than t₂ (theperiod of permanency t₂ corresponds to the flow rate value Q₂ whilemoving along the curve g, i.e. in the presence of a conventionalextrusion head) since, according to the present invention, the path forpassing from Q₁ to Q₂ is along curve S″. The extremes of curve S″ areused to calculate the elastic constant K of the resilient element 22.Therefore, according to the present invention, the period of permanencyincreases from t₁ to t′₂, the latter being smaller than t₂ and far awayfrom the critical value t_(scorch,2).

In FIGS. 2 to 4 further embodiments of the extrusion head according tothe present invention are shown.

The elements of the extrusion head which are structurally and/orfunctionally equivalent to those previously illustrated with referenceto FIG. 1 are indicated with the same reference numbers. The embodimentsshown in FIGS. 2 to 4 differ from that reported in FIG. 1 in that thedevice for adjusting the position of the female die 13 with respect tothe male die 12 along the longitudinal axis X-X comprises, in place ofthe resilient element 22, a servo-device 32 which detects the extrusionspeed variations (and thus the polymeric material flow rate and pressurevariations) and adjusts the position of the female die 13 with respectto the male die 12 along said longitudinal axis X-X on the basis of thedetected variations.

In particular, in the embodiments illustrated in FIGS. 2 to 4, theservo-device 32 comprises a pressure sensor 33 associated with theextrusion head 1 at the inlet duct 15.

The servo-device 32 further comprises a processing device 34 operativelyassociated with the pressure sensor 33. The processing device 34calculates the new positions of the female die 13 along the longitudinalaxis X-X in response to the variations detected by the pressure sensor33.

The servo-device 32 further comprises a device 35 for moving the femaledie 13 to the calculated new positions, said device 35 being operativelyassociated with the processing device 34.

In the embodiment illustrated in FIG. 2, the device 35 for moving thefemale die 13 to the new positions calculated by the processing device34 comprises a hydraulic actuator which includes a pump 40 and ahydraulic cylinder 45 reciprocally connected by means of a connectingduct 41.

The pump 40 is operatively associated with the processing device 34,while the hydraulic cylinder 45 comprises a stem 46 that is connected tothe female die by the interposition of a crank gear 47.

According to this embodiment, a position sensor 48 is associated withthe stem 46 of the hydraulic cylinder 45 with the processing device 34to detect the position of the stem 46 (this position corresponding to arespective position of the female die 13) and to send a correspondingelectrical signal to the processing device 34 to allow the latter tocalculate a possible new position for the stem 46.

According to a further embodiment of the present invention (not shown),the hydraulic actuator is replaced by a pneumatic actuator.Specifically, the pump 40 is replaced by a container of pressurizedfluid, while the hydraulic cylinder 45 is replaced by a pneumaticcylinder.

FIG. 3 shows a further embodiment of the extrusion head 1 of the presentinvention.

The elements of the extrusion head which are structurally and/orfunctionally equivalent to those previously illustrated with referenceto FIGS. 1 and 2 are indicated with the same reference numbers.

The embodiment shown in FIG. 3 differs from that of FIG. 2 in that thedevice 35 for moving the female die 13 to the new position calculate bythe processing device 34 comprises a linear actuator 55. The linearactuator 55 is provided with a driving means (not shown) which areoperated and regulated by a processor 50.

According to this embodiment, a position sensor 58 is associated withthe linear actuator 55 and with the processing device 34 to detect theposition of the female die 13 and to send a corresponding electricalsignal to the processing device 34 to allow the latter to calculate anypossible new position for the female die 13.

FIG. 4 shows a further embodiment of the extrusion head 1 of the presentinvention.

The elements of the extrusion head which are structurally and/orfunctionally equivalent to those previously illustrated with referenceto FIGS. 1, 2 and 3 are indicated with the same reference numbers.

The embodiment of FIG. 4 differs from that of FIGS. 2 and 3 in that thedevice 35 for moving the female die 13 to the new position calculated bythe processing device 34 comprises a gear electro-mechanical deviceincluding a gear mechanism 65 driven by an electric motor 66 and coupledwith the female die 13. The gear mechanism 65 which is coupled with thefemale die 13 is housed in a supporting element 23 of the typeillustrated and disclosed with respect to FIG. 1. The gearelectro-mechanical device further comprises a processor 67 whichoperates and regulates the electric motor 66. The processor 67 isassociated with the electric motor 66 and with the processing device 34.

According to this embodiment, a position sensor 68 is associated withthe electric motor 66 and with the processing device 34 to detect theposition of female die 13 and to send a corresponding electrical signalto the processing device 34 to allow the latter to calculate a possiblenew position of the female die 13.

With reference to the preferred embodiments of the extrusion head 1described above and illustrated in FIGS. 2 to 4, the method of thepresent invention for depositing by extrusion a polymeric material 2 onan elongated element 3 advancing within the extrusion head 1 along adirection A comprises the following steps.

In a first step, similarly to the method described above with respect tothe extrusion head 1 illustrated in FIG. 1, after having conveyed theelongated element 3 within the longitudinal cavity 19 of the extrusionhead 1, the polymeric material 2 is fed to the feeding duct 16 of theextrusion head through the inlet duct 15 by one or more extruder screws(known per se and not shown in the figures). The polymeric material 2 iscaused to flow into the conveying channel 20 through the distributionchannels 18.

In a second step, the pressure sensor 33 detects—preferably at apredetermined frequency value—the pressure at the inlet duct of theextrusion head (said pressure value being correlated to the flow ratevalue and the latter being, in turn, correlated to the extrusion speedvalue) and generates a corresponding electrical signal which is sent tothe processing device 34.

Once a pressure variation is detected, in a third step of the method ofthe present invention the processing device 34 calculates a new positionof the female die 13 along the longitudinal axis X-X on the basis of thedetected variation and sends a corresponding signal to the actuatordevice 35 which moves the female die 13 to the calculated new position,thus adjusting the cross-sectional area of the second portion 20″ of theconveying channel 20. As mentioned above, since this cross-sectionalarea is correlated to the pressure (and thus to the flow rate) of thepolymeric material flowing through the extrusion head, the possibilityof adjusting this area allows to extend the working field of theextrusion head. In other words, according to the present invention it ispossible to increase the range of variation of the flow rate of thepolymeric material flowing through the extrusion head while ensuringthat the other process parameters (in particular, temperature and periodof permanency) remain within acceptable ranges of values so thatcritical flow conditions (and thus scorches, overheating, stagnations ofthe polymeric material as well as mechanical damages of the extrusionhead) do not substantially occur.

The considerations given herein above with respect to the graphsreported in FIGS. 5, 6 and 7 apply also to the embodiments shown inFIGS. 2 to 4, with the only exceptions that the extremes of curves S′and S″, along which the female die 13 moves from position |X₁| toposition |X₃|, are used to calculate the calibration of the actuatordevice 35 instead of the elastic constant K of the resilient element 22of FIG. 1.

1-36. (canceled)
 37. An apparatus for extruding a polymeric material,comprising an extrusion head which comprises: a male die; a female diecoaxially arranged with respect to said male die; a conveying channel,at least one portion of which is defined between said male die and saidfemale die; and a device for adjusting a cross-sectional area of said atleast one portion of said conveying channel by reciprocally displacingsaid female die with respect to said male die in response to anextrusion speed variation of said polymeric material.
 38. The apparatusaccording to claim 37, wherein the male die is provided with an innercavity coaxially extending with respect to a longitudinal axis of theextrusion head.
 39. The apparatus according to claim 37, wherein thedevice for adjusting the cross-sectional area of the at least oneportion of the conveying channel comprises a resilient element acting onat least one die.
 40. The apparatus according to claim 39, wherein theresilient element is associated with the female die.
 41. The apparatusaccording to claim 39, wherein the resilient element is a spring whoseelastic constant K is selected in response to the range of variation ofthe extrusion speed.
 42. The apparatus according to claim 37, whereinthe device for adjusting the cross-sectional area of the at least oneportion of the conveying channel is a servo-device that detects thevariation of the at least one parameter indicative of the polymericmaterial flow conditions and adjusts said cross-sectional area inresponse to the detected variation of said at least one parameter. 43.The apparatus according to claim 42, wherein said servo-device comprisesa sensor acting on said polymeric material and generating a signalrepresentative of the variation of said at least one parameter.
 44. Theapparatus according to claim 43, wherein said sensor detects a pressurevariation of the polymeric material flowing through the extrusion head.45. The apparatus according to claim 42, wherein said servo-devicecomprises a processing device for calculating a new position of at leastone die.
 46. The apparatus according to claim 42, wherein saidservo-device comprises a device for moving said at least one die to anew position.
 47. The apparatus according to claim 46, wherein thedevice for moving said at least one die to said new position comprisesan actuator device.
 48. The apparatus according to claim 47, whereinsaid actuator device is associated with said at least one die and aposition sensor.
 49. The apparatus according to claim 47, wherein saidactuator device comprises a hydraulic device.
 50. The apparatusaccording to claim 47, wherein said actuator device comprises apneumatic device.
 51. The apparatus according to claim 47, wherein saidactuator device comprises a gear electro-mechanical device.
 52. Theapparatus according to claim 47, wherein said actuator device comprisesa linear actuator.
 53. A process for manufacturing a tyre, said processcomprising the steps of: forming a crude tyre on a supporting device;moulding said crude tyre; and curing said crude tyre, wherein the stepof forming the crude tyre comprises the step of extruding at least oneelastomeric material, the step of extruding comprising the steps of:feeding said elastomeric material to an extrusion apparatus comprisingan extrusion head, said extrusion head comprising: a male die; a femaledie coaxially arranged with respect to said male die; and a conveyingchannel, at least one portion of which is defined between said male dieand said female die; and adjusting a cross-sectional area of said atleast one portion of the conveying channel by reciprocally displacingsaid female die with respect to said male die in response to anextrusion speed variation of said elastomeric material.